1. Field of the Invention
The present invention relates to grain oriented ceramics and a production process thereof, a platelike powder for producing the same, and a thermoelectric conversion element. More specifically, the present invention relates to the following: grain oriented ceramics suitable as a thermoelectric conversion material constituting thermoelectric conversion elements used in a variety of thermoelectric generators (including solar thermoelectric generators, thermoelectric generators using temperature difference in seawater, thermoelectric generator using fossil fuels and regenerators using waste heat from factories or automobiles), accurate temperature control devices (including a photodetector, a laser diode, a field-effect transistor, a photomultiplier, a cell of a spectrophotometer and a chromatographic column), thermostats, air conditioners, refrigerators and electrical power sources for clocks; and a production process thereof; a platelike powder for producing such grain oriented ceramics; and a thermoelectric conversion element using such grain oriented ceramics as the thermoelectric conversion materials.
2. Description of Related Art
Thermoelectric conversion means direct conversion between electric energy and thermal energy, taking advantage of the Seebeck effect or the Peltier effect. The thermoelectric conversion has attracted attention as a technology for high-efficiency energy use since it is characterized as, for example: 1) discharging no excess of waste products during energy conversion; 2) allowing effective use of waste heat; 3) enabling electric power to be generated continuously until the materials deteriorate; and 4) dispensing with a moving part such as a motor or a turbine, thus being maintenance-free.
As an index for evaluating the performance of materials capable of converting between thermal energy and electric energy, namely, thermoelectric conversion materials, it is common to use a figure of merit Z (Z =S2"sgr"/K , where S, "sgr" and K are a Seebeck coefficient, electrical conductivity and thermal conductivity, respectively) or a dimensionless figure of merit ZT expressed as a product of the value of the figure of merit Z and the absolute temperature T at which that value is shown. The Seebeck coefficient represents the magnitude of thermoelectric power generated by temperature difference of 1 K. The thermoelectric conversion materials have their specific values of the Seebeck coefficient, and they are classified into those having positive Seebeck coefficients (p-type) and those having negative ones (n-type).
In addition, typically, the thermoelectric conversion materials are used in a state of joining between the p-type and n-type materials. Such a joining pair is commonly called a thermoelectric conversion element. The figure of merit of a thermoelectric conversion element depends on the figure of merit Zp of the p-type thermoelectric conversion material, the figure of merit Zn of the n-type thermoelectric conversion material, and the forms of the p-type and n-type thermoelectric conversion materials. It is known that, if the forms of those materials are optimized, the figure of merit of the thermoelectric conversion element increases with increasing Zp and/or Zn. Therefore, to obtain a thermoelectric conversion element having a high figure of merit, it is important to use thermoelectric conversion materials of which figures of merit Zp and Zn are high.
In these thermoelectric conversion materials, there have been known materials such as Bixe2x80x94Te, Pbxe2x80x94Te, Sixe2x80x94Ge and oxide-ceramic systems. Among them, a Bixe2x80x94Te system compound semiconductor exhibits excellent thermoelectric properties (ZT: approx. 0.8) near room temperature, and a Pbxe2x80x94Te system compound semiconductor does so in a middle-temperature range of 300-500xc2x0 C. However, these compound semiconductors are difficult to use in a high-temperature range. In addition, there is a problem that those materials include expensive rare elements (such as Te, Sb or Se) or highly toxic substances which place a load on the environment (such as Te, Sb, Se or Pb).
On the other hand, a compound semiconductor of a Sixe2x80x94Ge system exhibits excellent thermoelectric properties in the high-temperature range around 1000xc2x0 C., and its materials contain no environmentally hazardous substances. However, for prolonged use of the compound semiconductor of the Sixe2x80x94Ge system at high temperatures in air, it is required to protect the surfaces of its materials, which deteriorates the performances of the thermoelectric element.
In contrast to this, thermoelectric conversion materials of an oxide-ceramics system do not necessarily contain a rare element or an environmentally hazardous substance. In addition, their thermoelectric properties do not deteriorate greatly even if they are used for prolonged periods of time at high temperatures in air, meaning that they are excellent in heat endurance. Therefore, the thermoelectric conversion materials of oxide-ceramic systems have received attention as materials that can replace compound semiconductors, and there have been various propositions about new materials having excellent thermoelectric properties and about the processes for producing those materials.
For example, A. C. Masset et al. prepared a polycrystalline body and a single crystal of Ca3Co4O9 that is a kind of layered oxide containing cobalt (hereinafter referred to as a xe2x80x9clayered cobaltitexe2x80x9d), and they evaluated the crystal structure and thermoelectric properties (see A. C. Masset et al., Phys. Rev. B, 62(1), pp.166-175, 2000). This literature mentions that Ca3Co4O9 is a lattice misfit-layered oxide in which Ca2CoO3 layers having a rock-salt crystal structure and CoO2 layers having a CdI2-type crystal structure are stacked at a predetermined cycle along a c-axis.
In addition, the same literature mentions that specific resistance of Ca3Co409 is anisotropic; the specific resistance is much smaller within the {001} plane than in the direction perpendicular to the {001} plane (i.e. along the c-axis). Furthermore, it also mentions that the Seebeck coefficient in the direction of the {001} plane of the Ca3Co409 single crystal reaches approximately 125 xcexcV/K in the neighborhood of 300 K and that the Seebeck coefficient has small dependence on temperature.
The xe2x80x9c{001} planexe2x80x9d of the layered cobaltite denotes a plane having excellent thermoelectric properties, that is, a plane parallel to the CoO2 layer. Many kinds of the layered cobaltite have not been clarified concerning their crystal structures. Moreover, their crystallographic axes and crystal planes are defined variously depending on what unit lattice is adopted. Nevertheless, the {001} plane is defined as above in the present invention.
Also, Japanese Patent Application Unexamined Publication No. 2001-19544, for example, discloses a sintered complex oxide of which composition is expressed by such a general formula as Bi2Sr2xe2x88x92xCaxCo2Ow, Bi2xe2x88x92yPbySr2Co2O, w or Bi2Sr2xe2x88x92zLazCo2Ow (where 0xe2x89xa6xxe2x89xa62, 0xe2x89xa6yxe2x89xa60.5, 0 less than zxe2x89xa60.5) and which has a layered crystal structure and electrical conductivity of 1.0xc3x97104 S/m or higher. This publication also discloses a process of producing a complex oxide, in which to pelletize the powders including sources of Bi, Sr, Ca and Co, to heat the green body in oxygen with uniaxial pressing so as to partially melt part of the materials, and to make it cool slowly.
In addition, Japanese Patent Application Unexamined Publication No. 2000-269560 discloses a complex oxide assembly obtained by die-pressing NaCo2O4 crystals with 5 mm average grain size and 20 xcexcm average thickness which is synthesized by the flux method, and by hot-pressing the compact body. This publication also discloses a process of producing a thin film of a complex oxide by forming a NaCo2O4 thin film on a substrate using the sputtering method.
The layered cobaltite such as Ca3Co4O9 or Bi2Sr2xe2x88x92xCaxCo2Ow is a p-type thermoelectric conversion material having a relatively large Seebeck coefficient. Besides, its thermoelectric properties have anisotropy in accordance with its crystal orientation. Consequently, using a material in which a crystal plane having excellent thermoelectric properties (i.e. the {001} plane) is unidirectionally oriented allows maximum utilization of the anisotropy of thermoelectric properties, which can improve the figure of merit. In addition, it is expected that a thermoelectric conversion element using such materials may have a high figure of merit.
However, an ordinary ceramics production process, in which a mixture of simple compounds such as CaCO3 or Co3O4 containing ingredient elements is calcinated, molded and sintered, is incapable of providing a sintered body of the layered cobaltite in which crystal planes having excellent thermoelectric properties are oriented unidirectionally.
On the other hand, Japanese Patent Unexamined Publication No. 2001-19544 mentions that, if part of the materials is partially melted with the compact body uniaxially pressurized and the green body is then cooled slowly, recrystallization takes place in the cooling process, which brings forth a sintered body constituted of grains in which the {001} plane has developed in a direction parallel to the pressurized plane. This method, however, is limited to particular substance systems and compositions in which recrystallization may provide a desired crystal. This presents a problem, for example, that the method cannot be applied to any system that causes phase separation or crystal structure change during crystallization.
In addition, as disclosed by Japanese Patent Application Unexamined Publication No. 2000-269560, according to the sputtering method, a NaCo204 thin film with the {001} plane preferred with a high orientation degree may be formed on a substrate by optimizing substances of the substrate, sputtering conditions, and the like. However, only a thin film can be obtained by the sputtering method; it is difficult to produce thermoelectric ceramics having a cross-sectional area large enough for practical use. On the other hand, it is difficult to produce thermoelectric ceramics having a high orientation degree merely by hot-pressing the coarse platelike powder synthesized by the flux method.
Furthermore, it may be possible to crystallize a layered cobaltite as a single crystal for the purpose of orienting crystal planes that have excellent thermoelectric properties. However, there is also a problem that the production cost of single crystals is high. Moreover, it is usually difficult to prepare a bulk material of the order of millimeters in size for use in thermoelectric conversion, although small single crystals may be obtained.
The present invention has been made in view of the above circumstances and has an object to provide grain oriented ceramics which are composed of a layered cobaltite exhibiting excellent thermoelectric properties and which have a high figure of merit.
Another object of the present invention is to provide a production process of grain oriented ceramics, which is applicable to a wide range of ceramic systems regardless of substance systems, and which enables efficient production of a sintered body having a large cross-sectional area.
And yet, another object of the present invention is to provide a platelike powder suitable for producing such grain oriented ceramics.
Further, another object of the present invention is to provide a thermoelectric conversion element in which such grain oriented ceramics are used as thermoelectric conversion materials.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, grain oriented ceramics according to the present invention are constituted of a polycrystalline body of a layered cobaltite, and the {001} plane of each grain constituting the polycrystalline body has an orientation degree of 50% or more by the Lotgering""s method.
In the more preferred embodiment, a rocking curve full width at half maximum measured for the {001} plane of the layered cobaltite is, preferably, 15 degrees or less.
The grain oriented ceramics according to the present invention are constituted of the polycrystalline body of the layered cobaltite exhibiting excellent thermoelectric properties. In addition, the {001} plane of each grain in such ceramics is oriented at a high orientation degree. As a result, the figure of merit in a direction parallel to the orientation direction of the {001} plane takes on a larger value than that of a non-oriented sintered body of the same composition.
Further, the production process of the grain oriented ceramics according to the present invention includes a material preparation step of preparing a material including a first powder with a crystal plane A which has lattice matching with a CoO2 layer of a layered cobaltite, a molding step of molding the material such that the crystal plane A is oriented, and a sintering step of heating and sintering the green body obtained in the molding step.
In one preferred embodiment, the first powder may preferably be an anisotropically-shaped powder having the crystal plane A as its developed plane.
In another preferred embodiment, the first powder may preferably be a precursor of the layered cobaltite. In this case, the material preparation step may preferably be a step of preparing a material including the first powder and the second powder which reacts with the first powder thereby forming the layered cobaltite.
In still another preferred embodiment, the first powder may preferably be composed of the layered cobaltite.
Further, in another preferred embodiment, the molding step may preferably be a step of molding the material such that an average orientation degree of the crystal plane A may be 55% or more by the Lotgering""s method.
When the material including the first powder is heated at a predetermined temperature, the crystal plane A of the first powder is succeeded to as the {001} plane of the layered cobaltite. Accordingly, orienting the crystal plane A in the green body and heating it at a predetermined temperature provide grain oriented ceramics in which platelike crystals of the layered cobaltite with the {001} plane developed are unidirectionally oriented.
In particular, the crystal plane A can be oriented easily if an anisotropically-shaped powder with the crystal plane A as its developed plane is used as the first powder. In this case, it is possible to obtain grain oriented ceramics with the {001} plane oriented at a high orientation degree.
In addition, in the case of producing an oriented sintered body composed of the layered cobaltite, the orientation degree of the {001} plane of the layered cobaltite in the sintered body depends greatly on the orientation degree of the crystal plane A of the first powder contained in the green body. Therefore, setting the orientation degree of the crystal plane A at a certain value or greater may provide grain oriented ceramics in which the {001} plane of each grain is oriented at an extremely high degree of orientation.
Furthermore, a platelike powder for producing the grain oriented ceramics according to the present invention is composed of Co(OH)2 and constituted of a platelike powder in which the {001} plane has developed. Co(OH)2 allows easy production of a platelike powder in which the {001} plane has preferentially developed. Besides, the {001} plane of Co(OH)2 has excellent lattice matching with the CoO2 layer of the layered cobaltite. Therefore, using such a platelike powder as a reactive template makes it possible to easily produce the grain oriented ceramics according to the present invention.
Moreover, a thermoelectric conversion element according to the present invention is constituted of the grain oriented ceramics according to the present invention as thermoelectric conversion materials. The grain oriented ceramics according to the present invention have a higher figure of merit than a non-oriented sintered body of the same composition. Consequently, the thermoelectric conversion element using such ceramics shows a higher figure of merit than that using the non-oriented sintered body of the same composition.
Additional objects and advantages of the invention are set forth in the following description, are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the claims.